Instrumentation sensors operating on a principle of vibration of constrained actuator masses are known in the art. Unlike gyroscopic instrumentation sensors, these sensors require no rotating parts. Vibrating actuator masses may take a number of different configurations such as forks, bars, plates, rings, or cups using piezoelectric or electromagnetic operation.
The principle of operation underlying vibratory mass instrumentation sensors is relatively simple. If the mass is vibrated or maintained in oscillation along the same direction as its guiding structural constraint, the mass will not apply any force (other than its own weight) in a direction transverse to the guide. This remains true so long as the guide structure maintains a constant orientation in space. However, if a guide is forced to rotate about an axis at right angles to itself, the oscillating member will apply alternating or pulsating forces to the guide structure. In this case, the average magnitude of the force will be proportional to the angular velocity of the forced rotation. Such forces can be measured by sensors such as piezoelectric structures. The forces exerted by the oscillating actuator mass on the sensor structure causes measurable electrical potential signals to be developed on the faces of the sensor structure. These signals can then be measured and calibrated to the rate of turn of the sensor instrument.
A drive circuit is required for such vibratory mass instrumentation systems. The drive circuit establishes and maintains the vibrating or oscillatory state of the actuator mass at an optimum level throughout the operative period of the instrument. Generally in such instrumentation, it is most desirable to vibrate the actuator mass at the natural resonance of the desired oscillatory mode. However, there are several resonant modes that may be close in frequency to the desired mode in most configurations. The usual means of allowing only one frequency of oscillation to be driven is to include a phase locked loop in the drive circuit. The phase locked loop serves as a bandpass filter with a substantially constant phase relationship to the sensed drive motion. The typical phase locked loop is digital in nature and will provide a square wave output. This square wave drive signal is rich in higher harmonics and will excite higher frequency resonant modes, which will interfere with the desired oscillatory mode.
Early designs of drive circuits commonly used square wave signals at adjustable frequencies based on the sensed drive signal. This caused transitional current or voltage spikes in the control signal that could not be adequately suppressed through conventional filtering techniques. These were subsequently abandoned for the more smooth transition of triangular or sinusoidal drive signals. Additionally, improved mass structures such as monolithic crystals and subsequent cup-style oscillator gyros have led to improvements in minimizing the differences between the drive element and the sensing element.
Commonly applied designs known in the art provide the drive circuit feedback signal by using a phase locked loop to generate the oscillations from the drive sense signal. This oscillation signal is further processed by a wave shaping device such as a switched integrator, providing a triangular shaped drive signal. Although this method provides improved transitions between oscillations, there is still a significant chance of generating harmonics resulting in unwanted vibrations in the drive mechanism. Subsequent filtering techniques tend to add complexity, and cost to the design and are not very effective at eliminating the harmonics. Therefore, a need exists for an electronic circuit that can generate a smooth oscillatory drive signal from the drive sense signal while maintaining substantially the same phase relationship. In addition such a circuit should have relatively low complexity and low cost.
The present invention provides a solution to this and other problems known in the art, and offers other advantages over the prior art.